In automation technology, the most varied of field devices are applied for determining and/or monitoring at least one process variable, especially a physical or chemical, process variable. In such case, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, conductivity, etc. The respective measuring principles are known from a large number of publications.
A field device includes typically at least one sensor unit, coming, at least partially and at least at times, in contact with the process, and an electronics unit, which serves, for example, for signal registration, —evaluation and/or—feeding. Referred to as field devices in the context of present invention are, in principle, all measuring devices, which are applied near to the process and which deliver, or process, process relevant information, thus also remote I/Os, radio adapters, and, generally, electronic components, which are arranged at the field level. A large number of such field devices are produced and sold by the applicant.
Electromechanical transducer units are used quite often in field devices. For example, they are used in vibronic sensors, such as, for example, vibronic fill level- or flow measuring devices, however, also in ultrasonic, fill-level measuring devices or ultrasonic, flow measuring devices. To explore separately and in detail each and every such field device and its measuring principle, where an electromechanical transducer unit of the invention could be used, would be to go beyond what is necessary. Therefore, the description has been limited, by way of example, to fill level measuring devices with an oscillatable unit. Based on this description, it will be apparent to those skilled in the art how the invention can be applied in other areas of use.
The oscillatable unit of such a fill-level measuring device, also referred to as a vibronic sensor, is, for example, an oscillatory fork, single rod or membrane. The oscillatable unit is driven during operation by means of a driving/receiving unit, usually in the form an electromechanical transducer unit, to excite the oscillatable unit, such that it executes mechanical oscillations. Examples include piezoelectric, electromagnetic or also magnetostrictive driving/receiving units. Corresponding field devices are produced in great variety by the applicant and sold, for example, under the marks, LIQUIPHANT and SOLIPHANT. The underpinning measuring principles are fundamentally known. The driving/receiving unit excites the mechanically oscillatable unit by means of an electrical exciting signal to execute mechanical oscillations. Conversely, the driving/receiving unit can receive mechanical oscillations of the mechanically oscillatable unit and convert them into an electrical, received signal. The driving/receiving unit can be either a separate drive unit and a separate receiving unit, or a combined driving/receiving unit.
The most varied of both analog as well as also digital methods have been developed for exciting the mechanically oscillatable unit. In many cases, the driving/receiving unit is part of a feedback, electrical, oscillatory circuit, by means of which the exciting of the mechanically oscillatable unit to execute mechanical oscillations occurs. For example, for a resonant oscillation, the oscillatory circuit condition must be fulfilled, according to which the amplification factor is ≥1 and all phases arising in the oscillatory circuit sum to a multiple of 360°. This means that a certain phase shift must be assured between the exciter signal and the received signal. For this, the most varied of solutions are known. In principle, the setting of the phase shift can be performed, for example, by application of a suitable filter, or also be controlled by means of a control loop to a predeterminable phase shift, the desired value. Known from DE102006034105A1, for example, is to use a tunable phase shifter. The additional integration of an amplifier with adjustable amplification factor for additional control of the oscillation amplitude is, in contrast, described in DE102007013557A1. DE102005015547A1 proposes the application of an allpass filter. The setting of the phase shift is, moreover, possible by means of a so-called frequency sweep, such as disclosed, for example, in DE102009026685A1, DE102009028022A1, and DE102010030982A1. The phase shift can, however, also be controlled by means of a phase locked loop (PLL) to a predeterminable value. Such an excitation method is subject matter of DE00102010030982A1.
Both the exciter signal as well as also the received signal are characterized by frequency, amplitude and/or phase. Changes in these variables are then usually taken into consideration for determining the respective process variable, such as, for example, a predetermined fill level of a medium in a container, or also the density and/or viscosity of a medium. In the case of a vibronic limit level switch for liquids, for example, it is distinguished, whether the oscillatable unit is covered by the liquid or freely oscillating. These two states, the free state and the covered state, are, in such case, distinguished, for example, based on different resonance frequencies, thus a frequency shift, or based on a damping of the oscillation amplitude.
Density and/or viscosity can, in turn, be ascertained with such a measuring device only when the oscillatable unit is covered by the medium. Known from DE10050299A1, DE102006033819A1 and DE102007043811A1 is to determine the viscosity of a medium based on the phase frequency curve (ϕ=g(f)). This procedure is based on the dependence of the damping of the oscillatable unit on the viscosity of the medium. In order to eliminate the influence of density on the measuring, the viscosity is determined based on a frequency change caused by two different values for the phase, thus by means by a relative measurement. For determining and/or monitoring the density of a medium, in contrast, according to DE10057974A1, the influence of at least one disturbing variable, for example, the viscosity, on the oscillation frequency of the mechanically oscillatable unit is ascertained and compensated. In DE102006033819A1, it is, furthermore, proposed to set a predeterminable phase shift between the exciter signal and the received signal, in the case of which effects of changes of the viscosity of the medium on the mechanical oscillations of the mechanically oscillatable unit are negligible. At this phase shift, an empirical formula for determining the density can be established.
The driving/receiving unit is, as already mentioned, as a rule, embodied as an electromechanical transducer unit. Often, it includes at least one piezoelectric element in the most varied of embodiments. Using the piezoelectric effect, a high efficiency can be achieved. In such case, meant is the efficiency of the changing of the electrical energy into mechanical energy. Corresponding piezoceramic materials based on LZT (lead zirconate titanate) are, normally, suitable for use at temperatures up to 300° C. There are piezoceramic materials, which keep their piezoelectric properties at temperatures above from 300° C.; these have all, however, the disadvantage that they are significantly less efficient than the materials based on LZT. For use in vibronic sensors, these high temperature materials are, moreover, only conditionally suitable due to the large differences in the coefficients of thermal expansion between metals and ceramic materials. Because of their function as force providers, the at least one piezoelectric element must be connected for force transmission to a membrane, which is part of the oscillatable unit. Especially in the case of high temperatures, however, often large mechanical stresses are experienced, which can lead to a breaking of the piezoelectric element and, associated therewith, a total failure of the sensor.
An alternative, which can be better suitable for use at high temperatures, is represented by the so-called electromagnetic driving/receiving units, such as, for example, described in WO 2007/113011 and WO 2007/114950 A1. The changing of electrical energy into mechanical energy occurs, in such case, via a magnetic field. A corresponding electromechanical transducer unit includes at least one coil and a permanent magnet. By means of the coil, an alternating magnetic field is produced passing through the magnet, and via the magnet a periodic force is transmitted to the oscillatable unit. Usually, the transmission of this periodic force occurs via the solenoid principle, where the mobile core contacts the membrane centrally.
Since in the case of an electromagnetic driving/receiving unit, no force transmitting connection with the membrane of the oscillatable unit is necessary, such can be used, in comparison with piezoelectric transducer units, in an expanded temperature range, especially between −200° C. and 500° C. However, as a result of the absence of a force transmitting connection, usually the efficiency is significantly less than in the case of piezoelectric driving/receiving units. While an electromagnetic driving/receiving unit can develop relatively high forces in the region of the membrane, the deflection of the oscillatory fork is comparatively small as a result of there being no force transmitting connection between membrane and drive. As a result, more energy is required for an electromagnetic driving/receiving unit in comparison to a piezoelectric driving/receiving unit, a situation which makes use of a corresponding sensor in explosion-endangered regions problematic.